[vslib] MACsec interface creation command fails on VM/VS with send_sci=false and SCI combination.

[vikram-nexthop]

What I did

Fix for #1667

How I did it

Modified MACsec interface creation logic to use a two-step approach:

1. Always create the MACsec interface with explicit SCI specification (default send_sci=on)
2. When send_sci=false is required, add a second command to switch the interface to send_sci=off mode.

How to verify it

1. Run MACsec tests with profile parameted send_sci set to false.
2. Verify interface creation commands in syslog:

2025 Oct 15 16:26:50.911039 VM01T3 NOTICE syncd#syncd: :- create_macsec_egress_sc: /sbin/ip link add link "eth1" name "macsec_eth1" type macsec  sci 226b54b065000001 encrypt  on  cipher GCM-AES-128 && ip link set link "eth1" name "macsec_eth1" type macsec  send_sci off && ip link set dev "macsec_eth1" up

3. Check that TX SCI and send_sci are correctly set using command 'ip macsec show'

admin@VM01T3:~$ ip macsec show
83: macsec_eth1: protect on validate strict sc off sa off encrypt on send_sci off end_station off scb off replay off 
    cipher suite: GCM-AES-256, using ICV length 16
    TXSC: 226b54b065000001 on SA 0
        0: PN 1534, state on, key 8912df4fd8ea3f1547e02a4410f89996
    RXSC: 5c0758de98020001, state on
        0: PN 1597, state on, key 8912df4fd8ea3f1547e02a4410f89996
    offload: off 

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